Merge branch 'pendo-poc' into 'master'

Backport: Pendo POC snippet

See merge request gitlab-org/gitlab-ce!29439

app/views/layouts/_head.html.haml

@@ -78,3 +78,4 @@
   = render 'layouts/google_analytics' if extra_config.has_key?('google_analytics_id')
   = render 'layouts/piwik' if extra_config.has_key?('piwik_url') && extra_config.has_key?('piwik_site_id')
   = render_if_exists 'layouts/snowplow'
+  = render_if_exists 'layouts/pendo' if Feature.enabled?(:pendo_tracking) && !Rails.env.test?